In the context of epistemology, the reliability of a method of gaining knowledge refers to the extent to which the method can be depended on to produce true results.

Science is often put forth as the most reliable way of developing an understanding of the world around us because its conclusions are based upon, and continually checked against, observedfacts.

Esoteric ways of knowing such as divine revelation are, on the other hand, quite unreliable, as can be seen by comparing the different conclusions reached by different people or groups purporting to receive revelation from the same divine source. (See, for example, the many denominations of Christianity.)

In order to determine whether a method of knowledge gathering is reliable, one would need to tally all the individual claims, and see how many of them have been confirmed. Unfortunately, those who claim to have esoteric knowledge are often found to be victims of different problems, such as confirmation bias. Many of these claims turn out to be unfalsifiable or non-testable, so there's ultimately no way to evaluate the claims. Through science, many of these issues are resolved.